A “flip chip” refers to an integrated circuit that includes at least one semiconductor die, which is bonded circuit-side down to a substrate, with direct electrical interconnection between the die and the substrate. Because the die is connected directly to the substrate, traditional bond wires are eliminated. The substrate can be a passive carrier such as a printed circuit board, or it can be another semiconductor chip. The substrate is normally bonded directly to a motherboard. Other flip chips and other integrated circuits employing a variety of more traditional packages such as lead frame packages, surface mounts, pin grid arrays and the like can also be mounted to the motherboard.
One purpose that the substrate serves is to allow the input-output (I/O) signals on the die to “escape” the die onto the motherboard. Dice are usually quite small, and contain as many as hundreds of I/O signals, as well as numerous power and ground connections. There can be “bumps” (i.e., solder spheres) on the surface pads of the die to facilitate electrical connections to the substrate. These bumps are densely packed together onto the small die. It may not be practical to attempt to bond such tightly packed bumps to a motherboard. The substrate serves the purpose of spreading out these densely packed bumps to a much less dense spacing, so that these I/O signals and power and ground connections can then be connected to the motherboard.
When the die is bonded to the substrate, the bumps on the die are the points of physical and electrical contact between the die and the substrate. The bumps carry electrical signals including power and ground to and from the die. The substrate has a surface, typically the surface opposite the die, which has a plurality of contacts called pads or lands. A solder ball is typically attached to each land for soldering to the motherboard. The solder balls are collectively referred to as a ball grid array, because they are usually arranged in a grid pattern.
Each bump in the die bonding area is electrically connected to a corresponding ball in the ball grid array through conductive segments called “traces” along one or more of the layers in the substrate and through one or more “vias” between the layers.
The I/O signals being routed through the substrate for communication between the die and motherboard can include many different types of signals, such as single-ended signals and differential signal pairs. A differential signal pair consists of two adjacent I/O contact pads within the die bonding area, which are routed to two adjacent balls in the package. Ideally, the routing for the two traces in each differential pair should be identical in terms of the length of the routes, the number of vias in the routes and the planes between which the traces are routed. However due to the density of contacts within the die bonding area and limitations of the substrate routing layers, it may not be possible to match both routes for the entire distance of the routes.
A large length difference between two traces in a differential signal pair increases the propagation delay in the longer trace, which should ideally be the same along both traces. In addition, jogs in one trace relative to the other trace increases the impedance of the jogging trace.
Improved routing schemes are desired for routing differential signal pairs along a flip chip substrate.